On 24 and 25 September 2009 the dust which had so dramatically affected Sydney and the South East Coast of Australia crossed the northern half of the North Island of New Zealand. Unusually, because of the particular weather patterns at the time, the dust arrived from the north east. As a result of this the national threshold concentration for PM10, of 50μgm-3 (24 hours average) was exceeded. PM10 is a term used to describe dust particles 10μm in size or less. They are small enough to enter the bronchi of the lungs and have been known to cause respiratory problems.

Peaks in concentration at Whangarei were greater than 250μgm-3 and were at 350μgm-3 at the Khyber Pass dust monitoring station in Auckland. Within the Waikato region peak dust concentrations were variable but did generally decline with latitude going south. At Ngaruawahia they were approximately 300μgm-3 and 250μgm-3 at Hamilton. A clearer trend could be seen with 24 hour averages which dropped from 113μgm-3 at Ngaruawahia to only 16μgm-3 at Turangi.

Within Northland PM10 concentrations started to increase at around 6pm at Kaitaia and 9pm in Whangarei on the evening of 24 September. They peaked for about 3 hours and had dropped to normal background concentrations by 8am and 11am the following morning respectively. In Auckland concentrations started to increase at around 9pm on the 24th, peaked for about 4 hours and returned to background levels by noon on the 25th. In the Waikato the concentrations of dust started to increase around midnight, on the 24th, in Hamilton but not until 8am on the 25th in Putaruru and Tokoroa. They peaked for approximately 5 hours around Hamilton and had returned to normal by 3pm on the 25th. Further south, in Tokoroa, the dust concentrations peaked for 2 hours and were at background levels by 5pm.

To determine any health effects, from the dust, respiratory admissions to individual hospitals across Northland, Auckland and Waikato for September 2009 were examined. Although PM10 has also been associated with cardiovascular disease this relationship was not examined in this instance. Only at the Waikato Hospital in Hamilton was any effect evident. In all other areas no increase in respiratory disease admissions were apparent, either as individual diseases or when grouped to those that may be aggravated by dust inhalation. However on 27 September 2009 (2 days after the dust had passed) there were 10 asthma admissions (diagnostic code J459, asthma unspecified) at The Waikato Hospital. The mean number of admissions for the month was 2.23 per day with a 95% confidence interval of 1.56 to 2.9.

With higher PM10 concentrations and a higher population it would be expected that Auckland would be the most likely area to have demonstrated any effect on health of the dust. It is interesting that the only area which seems to have witnessed any effect is the Waikato.

Although dust concentrations were still high in the morning they had peaked in Northland and Auckland before most people were out and about. In the Waikato, however, particularly south Waikato, the dust concentrations were still at or close to their peak when people started their day on the morning of 25th September. The dust was not particularly apparent, at least in Hamilton, (although some people did witness dust settling on their cars during the day) and the authors do not believe the increased admissions resulted from a psychosomatic effect from seeing the dust.

This small observation supports advice that people, especially those with asthma or other respiratory diseases, should stay in doors when PM10 concentrations are high.